The present invention relates to methods and devices for the wireless transfer of measurements made during chemical-mechanical planarization of semiconductor wafers.
Chemical-mechanical planarization (xe2x80x9cCMPxe2x80x9d) processes remove material from the surface of a semiconductor wafer in the production of integrated circuits. FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 27, and a planarizing liquid 28 on the polishing pad 27. The polishing pad 27 may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be a new generation fixed abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid 28 may be a conventional CMP slurry with abrasive particles and chemicals that etch and/or oxidize the wafer, or the planarizing liquid 28 may be a planarizing solution without abrasive particles that contains only chemicals to etch and/or oxidize the surface of the wafer. In most CMP applications, conventional CMP slurries are used on conventional polishing pads, and planarizing solutions without abrasive particles are used on fixed abrasive polishing pads.
The CMP machine 10 also has an underpad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the polishing pad 27. In one type of CMP machine, a drive assembly 26 rotates the platen 20 as indicated by arrow A. In another type of CMP machine, the drive assembly 26 reciprocates the platen 20 back and forth as indicated by arrow B. Since the polishing pad 27 is attached to the underpad 25, the polishing pad 27 moves with the platen 20.
The wafer carrier 30 has a lower surface 33 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 33. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 40 may be attached to the wafer carrier to impart axial and/or rotational motion (indicated by arrows C and D, respectively).
To planarize the wafer 12 with the CMP machine 10, the wafer carrier 30 presses the wafer 12 face-downward against the polishing pad 27. While the face of the wafer 12 presses against the polishing pad 27, at least one of the platen 20 or the wafer carrier 30 moves relative to the other to move the wafer 12 across the planarizing surface 29. As the face of the wafer 12 moves across the planarizing surface 29, the polishing pad 27 and the planarizing liquid 28 continually remove material from the face of the wafer 12.
CMP processes must consistently and accurately produce a uniform, planar surface on the wafer to enable precise circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-patterns to within a tolerance of approximately 0.1 xcexcm. Focusing photo-patterns of such small tolerances, however, is difficult when the planarized surface of the wafer is not uniformly planar. Thus, CMP processes must create a highly uniform, planar surface.
One problem with CMP processing is that the planarized surface of the wafer may not be sufficiently uniform across the whole surface of the wafer. The uniformity of the planarized surface is a function of several variables, including the pressure between the wafer and the planarizing surface, the temperature of the wafer and/or the planarizing surface, and the temperature and pH of the planarizing liquid. One conventional approach to addressing this problem has been to measure some or all of the above variables and adjust the CMP processing conditions to improve the uniformity of the wafers. This approach has created additional problems. For example, if the measurements are made while the CMP machine is stationary, they may not be representative of the actual conditions present during planarization. On the other hand, if sensors are placed on the wafer carrier to make measurements during planarization, mechanical means, such as slip rings and the like may be required to transmit electrical signals from the moving sensors to a stationary display.
One conventional approach for obtaining in situ measurements is to use remote sensing means. For example, an infrared gun may be used to measure the temperature of the wafer during planarization. This approach suffers from several drawbacks. One drawback is that the temperature readings obtained from the infrared gun may be distorted by the presence of the planarizing liquid. A second drawback is that remote sensing means may not be readily available for some types of sensors, for example, pressure transducers. Accordingly, it may be difficult to determine the pressure between the wafer and the polishing pad during planarization.
One conventional approach for obtaining in situ pressure measurements is to place the pressure transducer on a mechanical linkage between the wafer carrier and a fixed reference point. This approach may suffer from still further drawbacks. For example, the weight of the mechanical linkage may distort the pressure measurement, and the linkage itself may have such a high inertia that it is unable to respond quickly to sudden pressure changes.
Still a further drawback with the foregoing conventional approaches is that each approach may require that a sensor and associated peripheral hardware be installed on a large number of CMP machines, although the planarizing characteristics may need to be monitored only periodically. As a result, the cost for sensors, peripheral hardware, and maintenance may be higher than is necessary.
In the competitive semiconductor industry, it is also desirable to maximize the throughput of finished wafers. One factor that affects the throughput of CMP processing is the ability to accurately stop planarizing a given wafer or type of wafers at a desired endpoint. To determine whether a wafer is at its desired endpoint, conventional CMP processes typically stop planarizing the wafer and measure the change in thickness of the wafer with an interferometer or other distance measuring device. If the wafer is under-planarized, CMP processing is resumed and the wafer is periodically measured until the wafer reaches its desired endpoint. If the wafer is over-planarized, the wafer may be partially or fully damaged. The throughput of finished wafers is accordingly greatly affected by the ability to accurately and quickly determine the endpoint of individual wafers and/or types of wafer.
The present invention is directed toward a method and apparatus for the wireless transfer of measurements made during chemical-mechanical planarization of a semiconductor substrate with a planarizing device. The planarizing device may have a support, a platen assembly connected to the support, and a carrier movable relative to the platen assembly and the support to remove material from a semiconductor substrate positioned between the carrier and the platen assembly. In one embodiment, the apparatus may comprise a sensor connected to the platen assembly, the carrier, or the semiconductor substrate. The sensor generates a signal corresponding to a value of a selected property of the planarizing device or the semiconductor substrate. For example, the property may be a force exerted against the semiconductor substrate by the carrier, a temperature or resistance of the semiconductor substrate, or the pH of planarizing liquid surrounding the semiconductor substrate. The apparatus may further include a display spaced apart from the sensor and a wireless communication link coupled between the sensor and the display to transmit the signal from the sensor to the display. The wireless communication link may include an infrared, radio, or acoustic transmitter and receiver, or a pair of inductors.
In one embodiment, the signal may be transmitted in real time from the sensor to the display. In another embodiment, the signal may be stored and then transmitted in a batch manner, and the communication link may include a cable or the wireless means described above. In still another embodiment, the apparatus may include a feedback loop that changes the selected property based on the signal generated by the sensor.
In yet another embodiment of the invention, the apparatus may remove material from a substrate having a reflective layer and a transparent surface opposite the reflective layer. The apparatus may include a light source positioned to illuminate the substrate, and a light sensor positioned to detect the presence or absence of light reflected from the reflective layer through the transparent surface of the substrate. In a further aspect of this embodiment, the reflective layer may have a hardness approximately the same as the hardness of a semiconductor wafer so that removal of the reflective layer is representative of semiconductor wafer planarization.